I feel stupid here but, for some reason, can't get my transmission fill cap to snap back on after doing a transmission service. When I removed the cap I broke off the red tab and pushed the stem through with a small screwdriver and the cap came right off. When I try to replace it with a new tab it will not snap down. It seems like I'm missing something. The cap just slides right back off - the tab not engaging anything. The notch in the tab looks like it should engage the flange on the fill tube but is too far away.
What am I missing?

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'99 E300
'01 F250 PowerStroke